Conventionally, a signal compression technology in compliance with the MPEG (Moving Picture Experts Group) standard has been used in various fields and is applied to AV data recording and reproducing apparatus for instance.
FIG. 10 (a) is a diagram showing an example of general configuration of a decoder of audio/visual data (hereafter referred to as AV data) compressed in a form of an MPEG transport stream (hereafter referred to as MPEG-TS).
As shown in the diagram, in a stream decoder 100, a transport decoder 110 is means of receiving input of an MPEG-TS. Also, an AV data decoder 120 is means of receiving output from a transport decoder 110, and a frame buffer 130 is means of accessing an AV data decoder 120 and reproducing the AV data in a form capable of displaying on a display unit.
Next, FIG. 10 (b) is a diagram showing a procedure for decoding and reproducing an MPEG-TS with time as its horizontal axis, and FIG. 11 (a) is a diagram showing a configuration of an MPEG-TS, and FIG. 11 (b) is a diagram showing a configuration of an image stream in an AV bit stream. Hereafter, MPEG-TS decoding and reproducing operation by a stream decoder 100 will be described referring to the drawings.
First, if an MPEG-TS is inputted to the transport decoder 110, the transport decoder 110 detects any PAT from the inputted MPEG-TS as a step shown in (i) of FIG. 10 (b). Here, a PAT (Program Association Table) is a packet comprising, in an MPEG-TS, a program number assigned to a program recorded in the MPEG-TS and a PMT (Program Mat Table) showing an ID of a packet for transmitting a stream such as AV data composing the program, and PATs are scattered in an MPEG-TS as shown in FIG. 11 (a). Moreover, PMTs also exist as packets in an MPEG-TS.
The transport decoder 110 detects any PAT and then detects any PMT as a step shown in (ii) of FIG. 10 (b), and as the case may be, further detects any ID called CAT (Conditional Access Table) concurrently with detection of any PAT and PMT as a step shown in (iii) of FIG. 10 (b). While AV data used for pay broadcasting is scrambled so that a general viewer without a special purpose decoder cannot view it, a CAT shows an ID of a packet for transmitting decoding information for descrambling it, existing as a packet in an MPEG-TS.
The program specification information such as PAT, PMT and CAT is collectively called PSI (Program Specific Information), and so it can be said that operation of transport decoder 110 as shown in (i), (ii) and (iii) of FIG. 10 (b) is performing detection of PSI.
An MPEG-TS in which PSI is detected by the transport decoder 110 is inputted as an AV bit stream to the AV data decoder 120. The AV data decoder 120 decodes an image stream from the inputted MPEG-TS. As shown in FIG. 11 (b), in an MPEG bit stream, an image stream comprises the image frames of frame I, frame B and frame P, and of these image frames, the frame that must be decoded first is frame I. Thus, as in the steps shown in (iv) of FIG. 10 (b) and FIG. 11 (b), the AV data decoder 120 first detects a frame I. If the frame I is detected, with the frame I as a starting point, an image stream is decoded from the MPEG-TS (step (v) of the same Figure). The decoded AV data is outputted to a frame buffer.
The frame buffer receives input of AV data from an AV data decoder, and accumulates a certain amount of it (step (vi) of FIG. 10 (b)) and then outputs it to a display apparatus such as a display unit.
Incidentally, according to the above operation, it requires two seconds or so from a start of decoding an MPEG transport stream to actual display of AV data. This is caused by operation for decoding an MPEG-TS into ordinary AV data, and the time can be divided into two as follows. One is the time required for detecting PSI in a transport decoder (PSI waiting time), and the other is the time required for detecting the frame I in an AV decoder (frame I waiting time).
The time required for detecting PSI and the time required for detecting the frame I are felt by a user of an MPEG transport stream reproducing apparatus as waiting time from performing reproducing operation to actually becoming capable of viewing AV data, which has been problematic in terms of convenience.